Photolithography or optical lithography systems used in the manufacture of integrated circuits have been around for some time. Such systems have proven extremely effective in the precise manufacturing and formation of very small details in the product. In some photolithography systems, a circuit image is written on a substrate by transferring a pattern via a light or radiation beam (e.g., UV or ultraviolet light). For example, the lithography system may include a light or radiation source that projects a circuit image through a reticle and onto a silicon wafer coated with a material sensitive to irradiation, e.g., photoresist. The exposed photoresist typically forms a pattern that after development masks the layers of the wafer during subsequent processing steps, as for example deposition and/or etching.
Due to the large scale of circuit integration and the decreasing size of semiconductor devices, the reticles and fabricated devices have become increasingly sensitive to structure and process variations, such as overlay errors, critical dimension (CD) variations, film thickness and composition variations, etc. These variations, if uncorrected, can cause the final device to fail to meet the desired performance due to electrical timing errors. Even worse, these errors can cause final devices to malfunction and adversely affect yield.
Numerous techniques have been developed to measure various characteristics of semiconductor samples so as to improve yield. However, there is a continued need for improved targets, apparatus, and techniques for measuring characteristics of semiconductor samples.